1. Field of the Invention
The present invention generally relates to circuit board assemblies and, more particularly, to a circuit board assembly having a flat coil which forms an inductor.
2. Description of the Related Art
There is a circuit board assembly, as a drive circuit for a plasma display, which has a drive IC and peripheral circuit elements mounted to a single circuit board. Each of display elements of the plasma display is regarded as a capacitor when considered as a load, and charge and discharge are carried out when applying a voltage to each display element (capacitor) to perform a display. An electric power released from each display element when discharging is recovered through an inductor, and the recovered electric power is accumulated in a capacitor provided separately so as to use the accumulated electric power for charging a next display element (capacitor). That is, an electric power to each display element is recovered and reused through an inductor.
There is a flat coil element as the inductor used for the above-mentioned application. Such a flat coil element is a coil having a flat shape produced by laminating conductive wire layers and insulating layers according to a method for producing a printed wiring board, and a coil part formed in each layer is connected by vias (refer to Japanese Patent Laid-Open Patent Application No. 2002-280230)
Since the coil is formed in a flat shape, a magnetic field is generated in a direction perpendicular to a flat plane of the flat coil element. Therefore, in many cases, the flat coil element is mounted perpendicular to a circuit board in an upright state so that the magnetic field generated by the flat coil element is not influenced by pattern wirings of the circuit board.
On the other hand, there is suggested a method of mounting a transformer having a flat coil element to a circuit board in parallel. In such a method, a flat transformer formed by combining flat coils is connected to an edge of a circuit board so that the flat transformer is not overlapped with the circuit board (refer to Japanese Laid-Open Patent Application No. 5-304764).
In the above-mentioned circuit board assembly constituting a drive circuit for a plasma display, other circuit elements are mounted in the periphery of the drive IC. FIG. 1 is a perspective view of a circuit bard assembly constituting a drive circuit of a plasma display. In the circuit board assembly shown in FIG. 1, a power module 2, which is a drive IC, is mounted onto a circuit board 1, and other circuit elements are mounted in the periphery of the power module 2.
Generally, a flat coil element 3 is arranged on a load side, that is, an output side of the circuit board 1, and a large component part such as a capacitor 4 is arranged on an input side. Generally, the flat coil element 3 is mounted to the circuit board 1 so that a flat surface of the element is perpendicular to the circuit board 1 so as to miniaturize the circuit board 1 by reducing a mounting area on the circuit board 1. Additionally, since the power module 2 has a highly integrated drive IC 5 that generates a large amount of heat, a heat sink 6 as a heat radiator is provided to the circuit board 1.
In the above-mentioned circuit board assembly shown in FIG. 1, if the amount of heat generated by the power module 2 is increased with high-integration, a distance between the heat sink 6 and the flat coil element 3 is decreased. Since the flat coil element 3 generates a magnetic field, a part of the heat sink 6 is located in the magnetic field generated by the flat coil element 3 if the heat sink 6 is located in the vicinity of the flat coil element. The heat sink 6 is made of metal such as, for example, aluminum, and, thus, an eddy current may be generated in the heat sink 6 if the magnetic field generated by the flat coil element 3 is applied to the heat sink 6. Such an eddy current generates a loss of an electric power to be recovered by an electric current recovery circuit including the flat coil element 3.
Moreover, an eddy current is generated in a wiring layer and an electric power loss is generated even in a case where the flat coil element 3 is mounted onto the circuit board 1 in parallel but close to it since the wiring layer (pattern wiring) of the circuit board 1 is made of metal.